It is well known that IL-12 up-regulates synthesis of interferon-gamma (IFN-g) in activated T cells; however, the effects of IL-12 on expression of other cytokines are not well defined. In this project, we are examining the effects of IL-12 on production of multiple cytokines, including IFN-g, IL-2, IL-10 and tumor necrosis factor-a (TNF-a), by purified, normal human CD3+ T cells. Although resting T cells are largely IL-12-nonresponsive, anti-CD3-activated T cell blasts are highly IL-12-responsive as demonstrated by the ability of IL-12 to induce Stat4-mediated gamma-interferon response region (GRR) DNA-binding activity. We have found that activation of purified human T lymphoblasts on immobilized anti-CD3 mAb induces rapid expression of TNF-a mRNA, and a more gradual increase in mRNA levels for IL-2, IFN-g and IL-10. IL-12 markedly up-regulates expression of IFN-g and IL-10, and down-regulates production of IL-2. Inhibition of IL-2 production by IL-12 correlates directly with increased production of IL-10. Moreover, neutralization of IL-10 activity with anti-IL-10 antibodies normalizes IL-2 production in IL-12-treated T cells demonstrating that the inhibitory effects of IL-12 are IL-10-mediated. Thus, IL-12 simultaneously up-regulates production of IFN-g and IL-10, and, by a direct IL-10-dependent pathway, feedback inhibits production of IL-2. The fact that IL-12 differentially regulates synthesis of IFN-g and IL-2 in T cells demonstrates that IL-12 does not globally enhance expression of all Th1-type lymphokines. Furthermore, the ability of IL-12 to up-regulate production of IL-10 provides a mechanism for limiting IL-2-dependent clonal expansion of activated T cells, and defines a novel cytokine regulatory pathway that is inducible by IL-12. In future studies, we plan to further define the effects of IL-12 on cytokine production by activated T cells, and to compare the effects of IL-12 with that of other macrophage-derived immunoregulatory cytokines, particularly IL-1 beta and TNF-alpha. We will also further explore the functional consequences of increased IL-10 production in IL-12-treated T cells. The results of these studies will broaden our present understanding of the actions of IL-12 on immune effector cells. This information may be useful in interpreting any therapeutic activity induced by IL-12 in clinical trials now underway. It may also provide insight to the physiological basis for certain toxicities associated with high dose IL-12 therapy.